High definition television (HDTV) is now being introduced. One aspect is the transmission system known as the Grand Alliance Advanced Television (ATV) system. The transmission system employed is also known as the 8 vestigial sideband (8-VSB) digital transmission system. The ATV VSB transmission system is known and need not be described in detail. By way of background, the 8-VSB system may operate in a broadcast mode with 8-level symbols (3 bits per symbol). Before transmission, data are received serially and randomized, forward-error-corrected (FEC) using a Reed-Solomon coding technique, interleaved and trellis coded. The format for this Transmission Layer signal is a data frame which includes synchronization signals.
A data frame for the Transmission Layer 8-VSB system includes two fields of 312 segments and two field synchronization segments, each of which are 832 symbols long. The first four symbols in each segment are segment synchronization symbols. Subsequent segments convey data formatted in the manner discussed above (randomized and coded). The data segments include the FEC codes. In the 8-VSB format, the field synchronization segments are placed at the beginning of each field (e.g., the first and 314th segments of a 626 segment frame).
In accordance with the Advanced Televisions Systems Committee (ASTC) standards, a digital television system can be implemented as a distributed transmission system (e.g., a Single Frequency Network or SFN), having a central data processor and a plurality of associated spatially diverse transmitters. One of the obstacles of implementing a distributed network is synchronizing the various transmitters to avoid straining equalizers at the receiving end with multiple delayed copies of the same signal. Generally, synchronization of most components can be achieved via a synchronization data embedded in the signal.
In general, synchronization information in a digital video signal along with a standard time reference (e.g., via a global positioning system (GPS) receiver) allow for synchronization of the majority of the components in the transmitters. The transmitters, however, contain Trellis coders and pre-coders that contain memory, such that the output of a given coder is dependant on prior states of the coder. Each transmitter contains the equivalent of twelve Trellis encoders, and the state of these Trellis encoders should be periodically synchronized across the transmitters to maintain their synchronous operation.
In a prior implementation of a synchronized transmitter arrangement, described in U.S. Patent Application 2002/0140867 by Weiss, synchronization of the Trellis coders is achieved by sending packets of reference information relating to the desired states of the Trellis encoders. Each packet contains thirty-six bits of information, three bits for each of the virtual Trellis coders associated with a given transmitter. The three bits for each encoder are inserted into the virtual coders to force the Trellis coders into the desired states. The appropriate state for each coder is determined from a reference exciter at the central data processor. Packets can be sent repeatedly over a given data frame to increase the likelihood that a valid reference packet will be received, as synchronization of the transmitters cannot be achieved without these packets in the Weiss system.
It will be appreciated that the Weiss system can require packets to be sent fairly frequently, decreasing the effective bit rate of the system. Further, the Weiss system requires a reference exciter at the central data processor, which adds expense and complexity to the system.